Field of the Invention
This invention relates to a process whereby the nucleation of ice within concrete is controlled. It is current practice to protect concrete against frost damage by introducing air voids, which are generated by adding surfactants with the cement paste. Those voids protect against one mechanism of damage (hydraulic pressure), but not against crystallization pressure. By introducing nucleating agents into the voids, ice can be forced to occur only in the voids, and this will further reduce frost damage.
Concrete, like all porous media, has the ability to retain and absorb moisture. Under freezing conditions, ice can grow within the concrete pores, leading to significant internal cracking of the cement matrix and/or scaling of the concrete surface. While the precise mechanisms of frost action are not known, concrete deterioration is believed to result from three important forces: crystallization, hydraulic and diffusion/osmotic pressures. These mechanisms are thought to produce flows of metastable water in the concrete pores that generate sufficiently high stresses to induce fracture of the cement matrix. To reduce the internal pressures, air-entrained voids are often placed within the cement matrix to provide escape boundaries for the flow of unstable water.
From experimental evidence, properly air-entrained concrete samples have given consistently good results in terms of the ASTM C 666 standard freeze-thaw tests. However, in practice, the technique of air entrainment has several disadvantages such as inconsistencies in spacing factors (means half-distance between voids) and uncertainties in bubble stability. Both issues have caused frequent discrepancies between expected and actual frost durability.
Numerous references in this area are discussed in the Detailed Description section of this application.
It is a primary object of this invention to protect concrete from freeze damage.
It is a further object of this invention to provide a simple, inexpensive, and easy to use method of protecting concrete from freeze damage.
It is another object of the present invention to add an effective amount of nucleating agent to a cementitious mixture to nucleate ice in concrete.
It is a further object of the present invention to provide a nucleating agent in concrete which can be added during mixing.
It is even a further object of the invention to provide porous ceramic or clay shells for air entrainment in concrete, and to provide a method of making such shells.
These objects and others are achieved by the method of protecting a cementitious mixture from freeze damage according to the present invention. The method comprises incorporating air into a cementitious mixture to form air pores in the cementitious mixture, including an air entrainment agent, and adding an effective amount of, preferably, metaldehyde, or an equivalent nucleating compound for nucleating ice in the air pores upon the freezing of concrete. The nucleating agent is added to the cementitious mixture during the normal mixing process. Other nucleating agents may be used. Preferably the air entrainment composition contains a surfactant. Because ice nucleating agents are hydrophobic, when mixed with a surfactant, which is normally used for forming air voids, the metaldehyde particles associate themselves with the surfactant and become incorporated within air voids formed in the concrete. Optionally, the air entrainment is achieved by using porous ceramic shells, which could be used alone or which could be impregnated with metaldehyde or another ice nucleating agent. Preferrably, the metadehyde consists of tetrameric units (CH3CHO)4, rather than polyacetaldehyde chains.